Magnetic compass



`I une 25, 1946, s. KR'AsNowETAL I 2,402,638

MAGNETIC COMPASS Filed Sept. 4, 1941 2 Sheets-Sheet 1 44 -will i152? /4I fT-TJ fr0/Mary MAGNETIC COMPASS `Filed Sept. 4,' 1941 2 Sheets-Sheet 2ylil, 43

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` and other vehicles.

Patented June 25, `194,6

s 4PATENT oEFlcE MAGNETIC COMPASS Shelley Krasnow, New York, N. Y., andJoseph M.

' S. Kaufman, Washington, D. C.

Y Application September 4, 12.341', Serial No. 409,574

s claims.' (ci. 335-222) This invention relates to a new type ofmagnetic compass, useful Vfor indicating the direction of the magneticmeridian. In the specific form described, the instrument is particularlysuitable for the steering of boats, ships, airplanes, tanks, In theprior art, it had been customary to make such compasses of the liquidimmersed type. The moving system in these compasses would be entirelyimmersed in a liquid which in some cases was a light oil, in other casesa mixture of alcohol and water. Experience extending back over manyyears indicates that suchcompasses are quiteundesirable. The use ofliquid requires a liquid-tight container, which is simple enough intheory but difficult to obtain in practice. Moreover liquid willgenerally have corrosive effects. Even normally non-corrosive liquidswill often decompose under the action of light, and heat and will giveend products which will cause corrosion. Furthermore, leakage or removalof the liquid from the container would oftenV subject the instrument tohighly undesirable conditions such Vas abnormally high pivot loads.Furthermore., a fluid immersed compass cannot be shipped in disassembledcondition, sinceit requires an instrument maker or someo ne of equalskill .to assemble it. On the other l'iand,` compasses employing ,noliquid may be shipped in disassembled condition and the moving systemput in place very easily before the instrument is to be used,` or afterit is finally mounted. Thus the shocks caused by transportation, whichare often far greater than those caused by ordinary use, will beavoided.

@The undesirable features of liquid have long been recognized, but theyhave been accepted as necessary'evils. Inlsome cases the liquid has beenthought desirable inA order'to buoy, or partially iioat, the movingsystem of the compass. The thought behind this was to remove as much aspossible of theweight upon the pivot and thus make the instrumentmoresensitive, and pro long the life of the pivot. When this constructionwas adopted, a float was normally used and this gave rise to furthertrouble. An awkward type of design had to be used to have the center ofbuoyancy of the iioat in the right position. Furthermore, the iioatoften developed leaks, rendering t-he instrument entirely inoperative.Moreover, the pivot of such an instrument would often be subjected todangerous loads. Even though the weight had largely been lifted oft thepivot, the moving system would still be responsive to acceleration andthe accelerational forces would be dependent upon the mass of the movingsystem rather than its weight. Thus,` even though the load on the pivotwas theoretically very light, actually, due to vibration andacceleration, considerable forces would be set up in the moving systemand transmitted to the pivot.

Liquid has also been tolerated in the belief that it is necessary fordamping. Use of liquid has been indicative of a virtual confession that'other systems of damping were not successful.

Although magnetically damped compasses have been described in the priorart, they have not4 been particularly.successful. For one thing, thedamping had not been s uiiicient, the result of this being that thecompass would continue to swing to and, fro when subjectedto externaldisturbances. Furthermore, the damping was not uniform and symmetrical,resulting in a certain type of oscillation being quite undarnped, Whileanother type would be relatively effectively damped. This resulted in asort of ,loping behavior. A still further defect was that the compasshad to be formed in the shape of a sphere, and to have a dome-shapedtop. This type is somewhat awkward to use when it is desired to View thecompass from above as is often the case in the navigation of watercraft. For the latter purpose, it is highly desirable to have aninstrument of generally fiat form,` with a attened top.v

It is further desirable to have thecard or indicating member in the formof a dial of generally flat shape to .be viewed from above. The compassshown herein combines the advantages of magnetic damping of compassesshown in the prior artat the same time allowing a construction whichpermits the use of a flattened form of card and container.

The present invention is intended to supply a new type of magneticcompass, which is entirely independent of fluid of the type of liquid,and which has an efficiently damped system, entirely obviating the needfor fluid. Moreover, by making the system light, and by other improveddesign it has been found possible to reduce the pivot load to such adegree that fluid is unnecessary to avoid undue load on thepivot.

It is an object of the invention to provide a new type of magneticcompass, with an efficient magnetic damping system.

It is a further object of the invention to supply a dry magnetic compasswith balanced damping about any horizontal axis.

It is a further object of the invention to supply a magnetic compasswith a light magnetic system.

It isa, further object of the invention to supvide a compass which willbe especially resistant to lateral oscillation.

It is a further object of the invention to provide an eiiicientmagnetically damped compassof flat form.

It is a further object of the invention to provide an eiicient magneticunit with a minimum leakage and minimum moment of inertia.

It is another object of the invention to proi vide. a magnet systemwitha high order of sym-Y metry..

It isa further object. of the invention to provide a. magnet system withthe center of gravity at a. desired heightL Itis a. further objectv ofthe invention to provide. a magnet system of materials of diierentcoercive forces and different permeabilities, so.

as to make. most eiective use of the magnetic material and to obtain ahigh magnetic moment relative. to the weight,. and relative to themovment. of. inertia of the moving system.

It is a further object of the invention to provide a magnetic. elementwhich has radially projecting poles, so that prolongations of thesepoles. will, pass through the vertical axis of the system.

Itis a further object ofthe invention to provide. means to be attachedto the moving system of acompass, the. means serving to increase themoment of inertia about a horizontal axis, withv relatively littleincrease of moment of inertia about a vertical axis.

Although a specific type of magnetic. compass has been shown, it will beobvious that the methodsv and advantages. of the. invention may begained in other instruments having similar construction and used. forrelated purposes..

The advantages of. the. present invention are gained by a design whichfollows new lines. A principle employed is that of using an absolutelysymmetrical magnetic system. Prior art instruments have used magneticsystems which had poles. extending in one direction. In thev presentcompass, by the use of four poles. arranged in the form of a cross, asystem is. obtained which is mechanically symmetrical' and thereforerelatively resistant to mechanical disturbances. By thev use of such asystem, the heaviesty part of the instrument is concentrated at thecentral point or close thereto, this point being the one about whichrotation takes place. In view of the fact that the. moment of inertia ofa mass is proportional to the square of its distance. from the point ofrotation, the more mass that can be concentrated near the center ofrotation, the better. It is obvious that some sort of hub arrangementmust be provided for any compass. By making the hub serve severalfunctions, the dead weight carried by the pivot on which, the movingsystem rests, can be reduced and. the moment of inertia. reduced., TheAinvention also involves the use of tapered members extending radiallyfrom the hub. The tapered form serves two important functions. First, itwill be realized that since magnetic lines of force leak. from thesurface of the magnet throughout its entire length, the material nearthe ends of the magnet will actually carry less flux than the materialnear the center. Less area will therefore be required at the terminalportions. By reducing the cross-sectional area toward the terminalportions, a magnet is obtained which is unusually ecient for. itsweight., and. which. further has an especially low moment of inertia.The form employed for each of the prongs or arms of. the magnet may beroughly conical. For the same Weight of material, a cone rotated about atransverse axis through its base Will have a lower moment of inertia`than a cylinder rotated about a transverse. axis through its base. Thisis readily evident when one considers that the lightest part of the coneis furthest from the axis of rotation. Since the moment of inertia isproportional to the square of the distance of a weight from the axis ofrotation, the intrinsic. low moment of. inertia of the cone will be.readily appreciated. Since the period of oscillation of. the compass isproportional to yr M where M is the magnetic moment andi I is the momentof inertia, it will be seen that the form described will4 give a shorterperiod of oscillation in a uniform magnetic eld, such as the earths eld.

But still greater advantages are obtained in the matter of damping. Thefour radially projecting prongs mounted in a.- conductingcup which litsclosely thereabout, present four highly concentrated bundles of magneticlines of force, which, moving in close proximity to the conductingmaterial, induce currents therein which tend to damp oscillations,whether about the vertical. -axis ofthe instrument or about anyhorizontal axis. In view of the symmetry of the system, balanced dampingwill be obtained. If only a single magnet were used, as is contemplatedin the prior art, a lateral oscillation taking place about. a horizontalaxis passing through the magnet would not be damped at all,` while anoscillation about an axis. at right. angles to the axis of the magnetwould be damped efciently. In. the present system, the damping will beef,- fective Whether the. oscillationis about thev axis of one magnet orthe other since the magnets are exactly similar and Will behave in thesame way. From this it can be seen that the damping about anyhorizontal. axis will be. the same as that for any other horizontal.axis, and 13er-A turb'ations of the system. will be efficientlydam-pecl, no matter what their direction.

Because of the exceptionally good damping obtained, the center ofgravity of the magnet system may be left rather high, since littlerestoring force is needed to bring it to a level position. By makingthis change, the magnet system can be made relatively unresponsive tolateral accelerations.

A further principle involved is that of making the card of as low momentof inertia as possi-ble. 'I'his is accomplished by using an especiallylight card structure. It is possible, in fact, to make the card andmagnet structure so light, that the total load on the pivot. will belittle greater than. the load in a fluidv immersedr compass employing aoat.

By the, use of the principles enunciated above andthe constructiondescribed herein, it has-been possible to obtain Compasses which arecritically damped. It is possible to overdamp the instrument, but thisis generally not desirable. As a matter of fact, the most desirablecondition has been found to be one where the instrument is slightlyunderdamped. With this construction, it will overswing slightly andreturn to its proper position. y

By the use of the principles enunciated above and the constructiondescribed herein it has been found possible to obtain a compass whichhas a period of oscillation of four seconds.' using a four inch card. Aconventional compass using liquid and having the same size card wasfound to have a period of oscillation of 20 seconds. The im portance ofthe short period of oscillation in rap.` idly moving craft and smallcraftoperating in a rough sea can be well appreciated.

i A further advantage obtained with the system described is that sinceliquid can be obviated, materials may be used in construction whichwould be entirely unsuitable for immersion in liquid. Thus, an ordinarypaper dial may be used, or oneof a plastic.` Luminous paint may furtherbe employed in both the moving system and the stationary index marks,without any necessity of protecting the luminous paint againstdestruction by the liquid. 'A further advantage is that a mechanicalarresting system, similar to that used in surveying instruments, may beutilized to lift the entire moving system off its pivot when not in use.Thus, advantage may be taken of a type of construction which isimpracticable in a fluid lled compass.

Other objects and advantages of the invention will be apparent from thedetailed description taken in conjunction with thedrawings, in which:

l Figure 1 shows a plan view of a compass made according to theinvention.

Figure 2 shows a lateral cross-sectional view of the compass shown inFigure 1, taken across the plane 2 2.

Figure 3 shows a partial lateral cross-sectional view taken across theplane 3-3.

Figure 4shows a bottom view of a typical magnet systememployed in thecompass. f

Figure' shows a lateral view of the syste shown in Figure 4.

Figure 6 shows a partial lateral cross-sectional view taken across theplane 6 6 of Figure 4.

Figure 7 shows a lateral cross-sectional view of the damping cup, 1

Figure 8 shows a view of an alternative magnet system, made in onepiece.

Figure 9 shows a' partial cross-sectional view taken across the plane9-9 of Figure 8.

Figure 10 shows a partial lateral cross-sectional view taken across theplane III-I0 of Figure 8, showing the detail of one of the arms.

Figure 11 shows a top view with a portion in section of a compositemagnet system.

`Figure 12 shows the magnet system shown in Figure l1, in partiallateral cross-sectional View, taken across the plane I2-I 2.

lFigure 13 shows a lateral cross-sectional view of one of the arms ofthe magnet shown in Figure l1, taken across the plane I3-I 3.

Figure 14 shows still another composite magnet system, giving a top viewand a partial lateral cross-sectional view.

Figure 15 shows a cross-sectional view of the magnet shown in Figure14,` taken across the Figure 16 shows a. partial cross-sectional View ofthe magnet shownin Figure 14, taken across the plane Iii-I6.

Figure 1'7 shows a lateral cross-sectional view of an'alternative typeof construction, similar to that shown in Figure 2.

I represents the casing or bowl of a compass. To this are attached knifeedges 2, 2, resting in seats 3, 3. The seats 3, 3, for the knife edgesare mounted in a gimbal ring 4, which in turn is provided with knifeedges 5, 5, These rest in seats in a stationary support. At the upperpart of casing I is a recessed portion 6, in which is fitted atransparent cover l. This is held in place by a retaining ring 8, whichis in turn held in place by screws or other fastening'means 9. Fastenedrigidly to the interior of the casing I are a number of upright portionsI0. These may be three, four, or more innumber. The uprights I0 areprovided with threaded holes into which are placed adjusting screws I I,provided with pointed ends I2. A massive damping cup I3 is mountedwithin theA projections, and is provided with'conical depressions I4,into which the pointed ends of thescrews II nt. The damping cup I3 ispreferablymade of some material having a high electrical conductivity. Asuitable material for this purpose is copper of high purity, such as thecommercial electrical grade. Other suitable materials are silver oraluminum. The central portion of the cup I3 is provided with acylindrical v hole I5. At the lower part of this is a reduced hole I6,which is threaded. The interior portion of the cup is provided with aspherical surface I'I, whose purpose will be hereinafter de-` scribed.The lower part of the cup is made preferably conical in shape asrepresented by portion I8. The purpose of this will also be describedhereinafter. The upper portion of the cup has a rim portion I9 whichslants outwardly, This is part of a cone, whose apex lies upon thecentral axis of the cup. The cup is cylindrical in form on its exterior,and is generally provided with a fiat bottom. Mounted centrally in thecup I3 is a member 20 having a cylindrical exterior, and a reducedthreaded portion 2I. This member ts closely into hole I6 in cup I3, andscrews into threaded portion I6. It is therefore restrained fromWobbling laterally, and` at the same time is adjustable vertically byvirtue of the threaded portion 2I engaging with the threaded hole. I6.At the upper part of memberZO is a cup-shaped portion 23, in which fitsa bearing 22. The bearing 22 may be of a suitable hard material, and ispreferably made of sapphire, with a small spherical indentation polishedtherein. Resting in cup 22 is a threaded member 24 made in the form of ascrew with a slotted head, and a sharp point. A'high carbon steel may beused for this purpose, with a suitably hard point engaging with jewel 22so as to give a relatively frictionless bearing. The pivot 24 is engagedin a threaded hole in a vertical upstanding portion 25. The portion 25is integral with the portion 26, constituting a central hub member. Intothe member 26 is fastened by means hereinafter described magneticelements 2l. A collar 28 rests on top of hub member 26. Above collar 28is the central portion of a dial assembly. This central portion isdesignated as 29, and is retained in pla-ce by a superimposed member 3U.

Member 30 is provided with a cylindrical central hole, the end portionof which is threaded internally. The external end portion is slightlyenlarged and is preferably provided with a knurled surface. The threadedportion engages with the threaded end of the vertical upstanding portion2'5. Thus, the superimposed member V3E! may be removed by unscrewing. Itmay also be loosened to permit the card member to be oriented properlyrelative to the magnet system, after which it may be tightened.Extending radially from central portion 29 are several arms, which maybe four in number and which are represented as 3I. These arms carry adial member 32, which is provided with indicia, graduations or numbers,as desired. These indicia represented as 33, may either be applied byprinting or may be cut entirely through the material of the dial 32,permitting light to enter from below. The latter structure is preferredin some cases since it removes weight from the exterior of the dial,where the moment of inertia is greatest. It will be .noted that the arms3| are bent in such fashion as to clear they cup I3, and it will furtherbe noted that the conical portion I3 of the cup 'I3 is so shaped as toavoid interference with the motion of the card 32, when the latter isnot in horizontal position. In order to avoid diiiiculties which aresometimes met with under severe shak-` ing-of the compass in service,the point of the pivot 24, the plane of the card 32, the bottom oi theknife edgesZ, and the bottom of the knife edges 5 all lie in one` plane,i

Several modes of construction are possible for the central portion andthe magnets 21, and these modications are shown in the various drawings.A typical form is shown in Figure 4. Here a central portion 26 isprovided with radially projecting arms- 21. The pivot 24 projectsthrough the central part of 26, and -a cup-shaped portion 34 isprovided.,v This cup-shaped portion is so designed that a limitedclearance is left between it and the cup 23 of member 20. This preventsdisplacement of the card and associated portions from their mountingunder violent motion. A suitable material is soit iron, Permalloy, orSoft steel. Attached to the central portion 26 are magnets 21, which maybe made of chromium, tungsten or cobalt magnetV steel. They are,however, made preferably of a magnet material of the type of Alnico.After assembly, the magnets are magnetized in place, with the polaritiesas shown in Figure 4. The resultant magnetic axis ofthe combination willlie midway between the t'wo north poles, passing through the center ofthe system. It will be noted that the magnets are bent downwardly, asshown more clearly in Figure 5. The purpose of this' is to aid inlowering the center of gravity of the moving System; If portions of themoving system are above the point of support at 22, the system will beunstable mechanically unless weight can be added below. This is done byhaving the prongs 21, 21 project downwardly. The members 21, areprovided with a shoulder portion, which fits into a recess in member 25.Since the permeability of 'soft iron or Permalloy is very high comparedto most permanent magnet materials, only a small cross-sectional areaneed be used for the portions 26. The area need be small compared to thearea of the Alnico, a magnet material which has a relatively lowpermeability. The magnetic element mounted centrally in the cup is somounted that the terminal portions of the magnet 21, almost touch thespherical surface I1 of the damping cup. The closer these terminalportions can be brought to the damping cup, the better the dampingaction will be. The portion I1 is made a portion of a sphere having itscenter at the end of the pivot 24, this being the point about which themoving system rotates. The conical portion I8 is provided with such aslope that the magnet in its extreme position under disturbance will liealong the bottom portion I8. Preferably, the card 32 should be so placedrelative to the transparent cover 1, that the `rim of the card willtouch the cover 1 before the magnets touch the bottom I8 ofthe dampingcup I3. The height of the member 24, should be such that there is asmall clearance between it and the transparent cover 1. This clearanceIshould be so small that if the moving system is accidentally removedfrom its support by extreme shaking or vibration, it will fall backagain into the cavity in bearing 22, without the possibility of fallingout of this bearing entirely.

Various modiiications of the magnet system are possible. One is shown inFigures 8, 9 and 10. Here a central portion 35 is made integral witharms 35. A central cored hole 31 is provided into which is pressed ametal` disk 3B. This is provided with a threaded hole 4I, into which isplaced a threaded pivot .member 39. This is locked in place by lock nut4u. It is understood that the insert 38 may be made in the shape of theupper portion of the member 26, previously described. The entireassemblage of central portion 35 and arms 36 may be cast of Alnico, orother magnet material. This form i's especially suitable for thismaterial because Of the diiculty of its machining. It may be cast in onepiece and used in the rough state, and only need be ground where extremeroughness is encountered, or to remove asymmetries and to balance theelement. For ease in molding and casting, the bottom portion of themember 35 and of the arms thereof, may be made fiat as shown in Figure10. The flat portion of the bottom permits especially easy molding. Theupper portion is provided with a rounded surface. In general, it may besaid that arms Such as 21 or 36 should be of circular cross section. Ithas been found that a circular cross section will give the minimumsurface for a given shape. However, in the case of the element shown inFigure 9, the circular 'section'. has been abandoned in the interest ofeasier molding. Here the flat portion, while it presents a theoreticallygreater surface, existsl for only a portion of the contour, and Iwilltherefore introduce comparatively little extra leakage. i

It will be noted that the portion 35, particularly as shown in Figure 8,is 'made heavier than the corresponding member 25 shown in Figure 5. Thereason for this is twofold. Portion 26 has to carry only magnetic fluxwhile the portion 35 as shown in Figure 8, actually serves to furnishmagnetomotive force. In addition, since it is of material having alowerv permeability than that of the soft iron used in member 26, it hasto be made heavier. However, the moment of inertia added by member 35being in thickened form as shown, is relatively small, since thisportion is near the center of rotation. A suitable set ofproportions forthe arms 26 and 36 respectively is: length approximately three timesthegreatest lateral dimension at the base thereof. Other dimensions may beused, a shorter one having been found desirable in many cases. This isparticularly true of the element shown in Fig. 9. In fact, the arms 36can be made extremely short, making the member almost that of a completering with Short projections.

Another modification shown in Figures l1, 12 and 13 is one in which thecentral piece 42 is made of magnetic material such as Alnico. WhileAlnico has been found preferable to most other magnet materials, suchmaterials as cobalt magnet steel, tungsten magnet steel', and chromium lFigure 17 is suitable.

magnet steel may be employed. The members 43 are made of some materialhaving a high permeability, such as soft iron or Permalloy. Furthermodification of this is shown in Figures 14,

r15 and 16, where the desi-gn has been modified to tral member 44. Anenlarged portion 41 is provided to carry the magnetic flux efficientlyfrom member 44. This is rapidly reduced to the dimension of the portionshown at 48. Since the iron has a much higher permeability than a magnetmaterial of the type of Alnico, only a relatively small area is requiredin the portion 48 to carry the flux provided by an area such as seen atthe portion 41. The member 45 then tapers downwardly near the terminalportion 49. In thismodication, while a length of the member 45, equal toapproximately three times the greatest lateral dimension is useful, ithas been found that muchV greater lengths are sometimes desirable.

In some cases, the angle of swing allowed by the type of compass shownin Figure 2 is insufficient. In other cases, when the magnets are almostout of the damping cup, as occurs with violent shaking, the damping isinsufficient. For either of these cases, the modification shown in Herethe damping cup is made of two portions, 50 and 5| respectively. Bothare made of material of high electrical conductivity. The cup 50 isprovided with a recessed portion 54, which is engaged by acircumferential shoulder 55, provided on the upper member 5|. The Acardassembly 52 is assembled on the central hubafter the member 5| has beenplaced between the magnets and the card assembly. The magnet system withits pivot is then put in place, and the cup lowered so that the portions54 and 55 engage. It will be noticed that the supporting members 56 arebent in arcuate form. 'I'he purpose of this is to allow the member toclear the portion 5|. As before, the card 32 is preferably in the sameplane as the pivot.

.The lmoving system of the compass may be made of very thin aluminumsheet formed with downwardly projecting portions. This will givestiffness and at the same time furnish a very light movingsystem. Asstated previously, it is of the greatest importance to have theindicating member or card as light as possible, and of as low moment ofinertia as possible. Sheet aluminum as thin as $11000 of an inch hasbeen found suitable for making this member when the outer diameter isfour inches.` Thin sheet magnesium or magnesium alloy of the type ofDowmetal, or alloys of magnesium and aluminum, or beryllium, orberyllium alloys may successfully be utilized for the moving member.Alternatively, a plastic may be used, a suitable one being a vinylresin.` This may be used in thin sheet form, and the overhangingportions of both the dial and the arms thereof being fashioned. Analternative structure which has been foundsuitable .for the cardassembly and the arms supporting this assembly, is to have the arms 3|made of thin aluminum tubing of the type used for making the pointers ofelectrical measuring instruments. These are riveted to a central washer,which is fastened about the central hub of the magnet system, and arefurther riveted to a card such as 32. It will be noted that the members30 and the pivot member 34 contained therein are rather long. A purposeis served by this. It is desirable that the moving system have a greatmoment of inertia about a horizontal axis. This will aid in avoiding theeffects of oscillation about the horizontal axis when the compass isshaken or otherwise disturbed. By making the members 24 and 30respectively of elongated form, and especially by providing an enlargedportion such as shown at the terminus of element 30, inertia can beadded for movement about horizontal axes passing through the pivotpoint.V However, these masses will have relatively little moment ofinertia about a vertical axis passing through the pivot member 24.

When the magnet system rotates, whether it be about a horizontal,vertical, or inclined axis, the tips thereof will pass close to thedamping cup and will induce eddy currents therein. These will oppose themotion of the magnet and will tend to damp this motion. The four magnetsacting simultaneously will be particularly effective. Furthermore. sincethe lines of force are concentrated or crowded in the vicinity of thetips of the magnet, the magnetic field which will pass through thedamping cup will be intensified and will serve to make the damping moreefficient. While magnet systems employing four poles have beendescribed, other systems are possible and effective. Thus, a magnetsystem employing three north poles and three south poles,.eachprojecting radially from a central hub, and spaced sixty degrees apart,will also operate and give good results. The number may further beincreased to any even number of poles. Thus, eight poles or ten polesmay also be utilized. However, it is found that the simplicity andefficiency of four poles and the symmetry of this arrangement make ithighly desirable.

The type of symmetry utilized in the magnet system is one which may bebest expressed by the terminology used in crystallography. It will beseen that if the magnetic element is held in its normal position, and isrotated at about a vertica1 axis, thev appearance will be the same asbefore. This process may be repeated four times before one has acomplete revolution of the` element. The type of symmetry may thereforebe described as fourfold, similar to the description of symmetry of anoctahedron rotated about its long axis. Further, the magnetic system mayhave a sixfold symmetry, in other words it may have prongs subtendingangles of 60 at the center. Thus, each sixth of a revolution about thevertica1 axis will restore the appearance of the element to its originalappearance. In a system having sixfold symmetry, three of the magnetswould be polarized with north polarity, the opposite three having southpolarity.4 An ad- Vantage yof this type over the 90 form shown wouldbethat it would behave in superior fashion when used with a compensatingsystem. In general, the terminology of crystallography may be utilizedto describe the properties of magnetic systems. It will be appreciatedthat the description of mechanical symmetry and magnetic symmetry willbe different because of the different polarityof the members. y

. In the prior art. it had been usual to utilize i compass magnets ofeither straight cylindrical aioaees f form, or of the form oi a ilatsheet, tapered in one dimension only. In the present invention,advantage is taken of the superior results obtained by tapering theelement in two dimensions. In other words, the prongs or the poles ofthe magnet are made in the form of pyramids, or better still, cones. Thetapering in two dimensions has the great advantage of reducing theamount of surface area for a given volume of material, which furthertends to reduce the leakage. This follows from the familiar fact thatleakage depends upon the surface presented. At the same time, there isan additional improvement due to the lowering of the moment of inertia.By providing a for-m tapered in two dimensions, the moment of inertia isreduced to less than half of its figure as compared with what would beobtained for tapering in one dimension. In general, it may be said thata circular cross section and an approximately conical form is the mostdesirable. The more nearly the elements approach this form, the moreeilicient in general, they will be magnetically,

It is to be noted that the conical bottom i8 of the damping cup I9serves an important function. Since it lies close to the magnets,currents will be induced in the said conical portion and this portionwill further aid in damping the motion of the magnets. 'I'his effectwill be most pronounced when the indicating member or card isdisplacedfrom its horizontal position, since at that time the prongs of themagnets will approach more closely to the conical bottom of the dampingcup. r-lhis is a particularly valuable feature, since the damping ismost likely to be necessary when a compass has been violently shaken andis displaced from its horizontal position. In effect, the damping cup soformed and the relation of the 'magnets theretoprovides a system, theefliciency of damping of which is greater at times of violent motionthan ordinarily. This is a highly desirable feature since theoreticallythe damping should be zero when the system is nearly at rest, and shouldbe ver-y highwhen the system is in violent motion- This condition isalsov aided by the general phenomena of electromagnetic damping, sinceelectromagnetic damping is virtually non-existent at very lowvelocities, and increases as vsome higher power of velocity. It shouldbe noted in this connection, that in the modication shown in` Figure 14,since the prongs are relatively thin, the'moment of inertia will not beas much aiected'by making them longer as would be the c-ase with theother modications. Advantagemay be taken of this to make these prongslonger' so that a given angularY velocity will give a proportionallygreater linear velocity at the tips ofthe magnet. This will increase theeiectivenessof damping. Thus, if extremely good dam-ping isV desired,the prongs will be made very thin at thel extremity to secure a lowmoment of inertia but would be m-ade quite long to secure the highvelocity required for most effective damping.

Y Anexamination of the form of the compass shown in Figure 2particularly will reveal that an instrument f generally ilattened formis provided. There are many cases where such a form is highly desirable,a usual one being where the compass is read from above by a helmsman.Furthermore, a compass having a spherical exteriorv would require acurved glass piece in place of the window l. This is expensive toreproduce, is vulnerable to breakage, being in exposed position, andmust be finished verycarefully.` i Any irregularities will resultrindistortions and other features which are visually annoying. On the otherhand, a window such as 'l can be cut out of plate glass at relativelylow cost. The flattened form has been made possible by the use oi' thedamping cup as shown, by the bending of the arms holding the card, andby the general methods oi mounting the magnet and card. This has beendone without'sacricing the desirable feature of having the supportingknife edges of the entire compass, the card, and the pivot all lie inone plane. Moreover, the parts as shown may be fabricated at relativelylow cost as compared with the parts necessary in a spherical type ofcompass. y

This application continues and extends the teachings in applicantsapplication, Serial No. 399,014, (Patent No. 2,350,402, issued June 6.1944) further exemplified in applicants co-pending application, (SerialNo. 506,748.

The scope of the invention is indicated by the appended claims.

We claim:

l. A magnetic element for orienting an indicating member in a shipcompass and the like, consisting of at least two tapered portions, withtheir bases pointed toward each other and the reduced portions pointingaway from the center of the members, the members further being t9.-pered in two dimensions, so as to provide a minimum surface for magneticleakage, and a reduced moment of inertia, thereby furnishing a magneticsystem which has a high. ratio of magnetic moment to moment of inertia.

2. A Vmagnetic orienting element for use in ship compasses and the like,comprising a central hub portion, and radially projecting arms, thecentral hub portion being made of magnetic material of high coerciveforce, and relatively low permeability, the radially projecting armsbeing made of material of relatively high permeability, the projectingarms 'being of generally tapered form, with the larger end nearest thesaid hub portion, Vthe area of the enlarged 'portion and the area of thereduced portion beingsubstantially in the ratioof the permeability ofthe saidpermeable material to that of the material of highy coerciveforce, the aforesaid combination providing a magnetic integer which hasa minimum moment of inertia per unit of magnetic moment.

3. In a magnetic compass, a magnetic system including magnetic elementsadapted to rotate about a point of support, acup. of conductingVmaterial surrounding the said magnetic eledamping cup for lateral swingsof the magneticA system.

4. A magnetic orientingV element rfor magnetic Compasses andthelikecomprising a centralY hub portion of cup-shaped form, means withinthe cup-shaped portion providing for pivoting of the element, means for.attachment of poles of material having different mechanical. andmagnetic properties, and pole pieces of diierent magnetic Zproperties..fastened to, the. last-named, means,v

13 the polev pieces being each double tapered in form the said polepieces being each in the form of individual bars of material, theaforesaid assemblage providing a pivoted unit having a high magneticmoment relative to moment of inertia.

5. In a magnetic compass, a moving magnetic system to orient anindicating element, a damping element, the said damping element beingformed of highly conductive material with a spherically formed portiontherein, having the center of the spherical portion at the point ofsupport of the magnet system contained therein, further having a bottomconical portion, the magnet system being adaptedto rotate relative tothe said spherical portion so as to induce damping currents therein, andfurther to rotate relative to the bottom conical portion, so a5 toinduce damping currents in the said portion, the sides of the saidconical portion sloping downwardly from the point of support at the saidmagnet system, whereby the magnet system will 14 rotate close to, butnot touch the said conical portion for lateral swings of the said magnetsystem, thereby providing complete damping of the moving system withoutdanger of contact therewith.

6. A magnetic orienting system for magnetic Compasses and the likecomprising a cup shaped member of generally polygonal form, made ofhighly permeable material, with pivot means mounted Within the said cupshaped portion, a plurality of separate magnetic members, having a highcoercive force relative to the material of the said cup, each memberbeing attached to a respective one of the polygonal faces of the saidcup member, so as to project radially relative to the center of the cup,the aforesaid combination providing a magnetic integer having a highratio of magnetic moment to moment of inertia.

SHELLEY KRASNOW. JOSEPH M. S. KAUFM'AN.

